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In a GitHub thread, Michael Voříšek and Kamil Tekiela mentioned that the PCRE2 function `pcre_match` can be used to validate UTF-8, and that historically it was more efficient than mbstring's `mb_check_encoding`. `mb_check_encoding` is now much faster on hosts with SSE2, and much faster again on hosts with AVX2. However, while all x86-64 CPUs support at least SSE2, not all PHP users run their code on x86-64 hardware. For example, some use recent Macs with ARM CPUs. Therefore, borrow PCRE2's UTF-8 validation function as a fallback for hosts with no SSE2/AVX2 support. On long UTF-8 strings, this code is 50% faster than mbstring's existing fallback code.